Cathodic process for the preparation of tetraalkyl lead compounds

ABSTRACT

A process for the cathodic alkylation of lead using ethyl chloride, methyl chloride, or mixture thereof as an alkylating agent and iodide ion as an electrochemical catalyst with a liquid-liquid junction of propylene carbonate and water wherein propylene carbonate is in the catholyte and water is in the anolyte.

United States Patent Kang Yang;

James D. Reedy; William H. llarwood, all of Ponca City, Okla.

Oct. 9, 1969 Nov. 23, 1971 Continental Oil Company Ponca City, Okla.

Inventors Appl. No. Filed Patented Assignee CATHODIC PROCESS FOR THEPREPARATION OF TETRAALKYL LEAD COMPOUNDS Tomilov et al., Zh. Obshch.Khim. 35(2)39l-3, CA. 63, 5238f Primary Examiner-F. C. EdmundsonAttorneys-Joseph C. Kotarski, Henry H. Huth, Jerry B.

Peterson, Jack N, Shears and Carroll Palmer ABSTRACT: A process for thecathodic alkylation of lead using ethyl chloride, methyl chloride, ormixture thereof as an aikylating agent and iodide ion as anelectrochemical catalyst with a liquid-liquid junction of propylenecarbonate and water wherein propylene carbonate is in the catholyte andwater is in the anolyte.

CATHODIC PROCESS FOR THE PREPARATION OF TETRAALKYL LEAD COMPOUNDSBACKGROUND OF INVENTION 1. Field of Invention This invention relates tothe preparation of tetraalkyl lead compounds and more particularly to anelectrolytic process wherein an alkyl chloride is reduced at a leadcathode.

2 Description of Prior Art Electrolytic processes for the production ofalkyl leads are known. For example, Calingaert and Mead disclosedtetraalkyl lead formation at a lead cathode by electrolyzing a catholyteconsisting of an alkyl halide in either alcoholic caustic (Calingaert inU.S. Pat. No. 1,539,297) or aqueous caustic containing casein (Mead inU.S. Pat. No. 1,567,159). Also, Silversmith et al. disclosed a processfor preparing tetraalkyl lead compounds by electrolyzing at a leadcathode, a solution of an alkyl halide in a normally liquid,nonhydroxylic catholyte (Silversmith in U.S. Pat. No. 3,197,392). Themost recent disclosure in this field appears to be a patent to Smeltzwhich discloses a process for producing tetraalkyl lead compounds at alead cathode in an electrolytic cell having a catholyte containing analkyl halide, a tetraalkyl ammonium monohalide current carrier and fromabout 1 to about 20 moles of water per mole of said current carrier(Smeltz in U.S. Pat. No. 3,392,093).

SUMMARY OF INVENTION According to this invention an alkyl chloride isreduced at a lead cathode in a catholyte containing propylene carbonateand in the presence of an iodide ion acting as an electrochemicalcatalyst and with an anolyte containing water.

DETAILED DESCRIPTION Tetramethyl and tetraethyl lead can be preparedelectrochemically either by anodic oxidation or by cathodic reduction.The existing anodic processes demand the use of very expensivealkylating agents such as Grignard reagent or alkyl aluminum compounds.In addition the required solvent systems are hazardous and lowconducting. These shortcomings greatly aggravate the final process cost.

In cathodic process, alkyl halides are usually employed as alkylatingagents:

4RX+Pb+4e Pb(R) :l- 4x: l where R denotes CH or C l-I and X denotes ahalogen. Here one at once encounters a fatal shortcoming which is commonto all of the presently patented processes. Even though the currentefiiciency is very good when alkyl iodide or alkyl bromide is employed,the efficiency with chloride is so low that in this latter case there isvery little, if any, chance of commercial success. Bromides and iodidesare very expensive; in addition, a bromine or iodine recycle route thatcan be readily adopted to electrochemical process is not known atpresent. As it stands now, then, none of the known cathodic process islikely to compete with the nonelectrochemical process in whichsodium-lead alloy is employed.

We disclose a new cathodic process which is free from the aboveshortcomings. Briefly, what is new in our process is:

a. The use of an electrochemical catalyst with which alkyl chloride canbe reduced with high current efficiency, and

b. The use of a new solvent system in which a stable liquidliquidjunction is established.

The electrochemical process for the production of alkyl leads andchlorine using alkyl chloride and lead according to the equation:

4RCl+PbL',Pb(R) +2Cl (2) poses some technological problems. These are:

A. One must find a proper cathodic solvent and cathodic conditions tocarry out the cathodic reduction (1). The required solvent propertiesare: high-dielectric constant, appreciable solubility of inertelectrolytes, and highreduction potential so that the solventsthemselves do not undergo reduction before reaction 1 occurs.

B. The anodic solvent should be inert to chlorine. Other requirementsare: high-dielectric constant, appreciable solubility of some saltcontaining chloride anion, and a high-oxidation potential so that theoxidation reaction:

2Cl- -2e Cl (3) proceeds before the oxidation of solvents themselvesoccurs.

C. Proper partitioning agents, such as ion permeable membrane, which areinert to both anodic and cathodic solvents, must be found to separateanolyte from catholyte.

The key concepts in our process are (1) the use of I ion as anelectrochemical catalyst, and the use of immiscible liquid-liquidjunction to separate catholyte from anolyte. As far as we are aware, theuse of these concepts in the electrochemical production of alkyl leadsis entirely new. The iodide ion can be added in the form of sodiumiodide or potassium iodide.

In propylene carbonate the following nucleophilic substitution reaction,

4C I-l Cl-l-4I +4Na 4C H l+4NaCll (4) occurs readily and cleanly. Asshown later, ethyl iodide in propylene carbonate can be reduced with 100percent current efficiency:

4C I-I I-+4e+Pb Pb(C H ),+4I (5 From (4) and (5 we note that the netresult is the reaction of alkyl chloride and Pb to produce alkyl leads.No net consumption of iodide ion occurs. In view of these facts, wepresume that the catalytic property of iodide ion in our process is dueto the nucleophilic substitution reaction.

As mentioned earlier, another key concept in our process is the use ofimmiscible liquid-liquid junction to separate anodic solvent fromcathodic solvent. The propylene carbonate-water mixture forms twoimmiscible liquid layers. The bottom phase is mostly propylene carbonateand the small amount of water is this layer which can be reduced to lessthan few percent by adding alkyl halide is not harmful in the cathodicprocess. The concentration of propylene carbonate in water layer verystrongly depends on the concentration of salts as well as the amount ofalkyl halide in the propylene carbonate layer. For example, in the purepropylene carbonate-pure water system the water layer contains as muchas 22 weight percent propylene carbonate. This can be, however, reducedto less than few percent by adding sodium sulfate. The important fact inour application is that once equilibrium is established the furtherdiffusion of cathodic and anodic solvent does not occur. Since water cannow be used as an anodic solvent, the generation and recovery ofchlorine poses no problem.

Even though propylene carbonate has been used before as an electrolyticsolvent, the possibility of using the immiscible liquid-liquid junctionin electrochemistry has not been explored. lmmiscible liquid junctioncannot be set up between propylene carbonate and alcohols which aretypical hydroxylic solvents but is readily set up with water. Example 1below demonstrates the use of the above two key concepts in theproduction of alkyl lead.

EXAMPLE 1 A Pyrex cell with glass frit divider was used as anelectrochemical cell. The catholyte composition was: Propylenecarbonate-100 g., C H Cl--20 g., and Nal50 g. This mixture was heated to60 C. for 6 hours, and white precipitate was filtered off. More Nal wasadded to saturate the solution. Fifty cubic centimeters of the resultingsolution was charged in the cathode compartment. A flat Pb cathode hadan approximate area of 3 cm. Anolyte composition was: 100 g. water, 10g. NaCl, and 15 g. Net- Carbon anode was employed. The whole cell wasthen blanketed with 5 psi. CH and electrolysis was carried out at acontrolled constant current of ma. for 4 hours. The TEL formed wasextracted with hexane and was analyzed chromatographically. Results:

moles TEL formed =3.99X 10- Percent current efficiency 107 grams Pblost=0.7826

Percent current efficiency 101 The error involved in chromatographicanalysis undoubtedly led to the percent efficiency greater than 100percent; in any case, TEL yield is nearly quantitative. In thisexperiment, the sodium sulfite in the anode was used to scavange C1 Itis obvious that C1 can be recovered as a gas, or may be reacted in situwith various other compounds, such as ethylene.

We also made several attempts to produce TEL without using 1" catalyst,but otherwise under the same experimental conditions as in example I. InC I-I Br-LiBr system, TEL yield was only 40percent; this is too low tobe of any commercial interest. In C l-l Cl-LiCl system, TEL yield wasnegligibly small. These results clearly indicate that iodide ion is anindispensable catalyst.

EXAMPLE 2 The composition of catholyte in this was propylenecarbonate-lOO g., C H I-2O' g., NaI-lO g. while the anolyte compositionwas H l00 g., Nall0 g., Na SO -l5 g. Other details are given in exampleI. The result: percent current efficiency based on TEL formed 103.5.

EXAMPLE 3 systems employed before.

Having this described the invention by providing specific examplesthereof, it is to be understood that no undue restrictions orlimitations are to be drawn by reason thereof and that manymodifications and variations are within the scope of the invention.

What is claimed is:

1. In an electrolytic process for producing tetraalkyl lead compounds ata lead cathode in an electrolytic cell having a lead cathode, an anodeof a material which is resistant to attack by halogens, and acurrent-permeable partition separating the catholyte from the anolytewhich process comprises:

A. passing an electrolyzing direct current through a liquid catholytecomprising a. an alkylating agent of ethyl chloride, methyl chloride ormixtures thereof, b. propylene carbonate, c. an alkali metal iodide; andthrough a liquid anolyte comprising 1. water, and 2. a current carryingsalt; and B. recovering tetraalkyl lead from the catholyte; theimprovement comprising heating said liquid catholyte solution to about60 C. for about 6 hours prior to passing said electrolyzing current.

2. The process of claim 1 wherein said alkali metal iodide is sodiumiodide or potassium iodide.

3. The process of claim 1 wherein said current carrying salt in theanolyte is sodium chloride or potassium chloride.

4. The process of claim I wherein said alkylating agent is ethylchloride.

5. The process of claim 1 wherein said alkylating agent is methylchloride.

2. a current carrying salt; and B. recovering tetraalkyl lead from thecatholyte; the improvement comprising heating said liquid catholytesolution to about 60* C. for about 6 hours prior to passing saidelectrolyzing current.
 2. The process of claim 1 wherein said alkalimetal iodide is sodium iodide or potassium iodide.
 3. The process ofclaim 1 wherein said current carrying salt in the anolyte is sodiumchloride or potassium chloride.
 4. The process of claim 1 wherein saidalkylating agent is ethyl chloride.
 5. The process of claim 1 whereinsaid alkylating agent is methyl chloride.